Enhanced recovery of petroleum oil with water injection is a well-known technique to keep pressure at levels such that oil production is increased. In order to apply such method in oil fields injection water is generally sea water, taken in the vicinity of the platform and treated by de-aeration and filtration. Upon contact of the formation water which naturally exists below the oil zone with injection water, chemical reactions result which ultimately yield deposits of precipitated salts. In view of the fact that the formation water is rich in alkaline-earth metals such as Ba.sup.++, Sr.sup.++ and Ca.sup.++ and the sea water used as injection water contains sulfate ions, formation and deposition of insoluble compounds such as barium sulfate (BaSO.sub.4) and strontium sulfate (SrSO.sub.4) is mandatory, calcium sulfate (CaSO.sub.4) being less frequent. As a result from the thermodynamic imbalance of the carbon dioxide/bicarbonate system (CO.sub.2 /HCO.sub.3.sup.-) calcium carbonate (CaCO.sub.3) is also formed.
Scales resulting from the deposition of insoluble compounds are consistent and may cause plugs in valves, other equipment and production strings, with the consequent oil losses. Scales in heat exchange systems cause lower heat transfer rate to the cooling water, leading to overheating. Frequent cleaning lowers the useful life time of equipment and increase the production cost of petroleum oil.
Further, in view of the fact that in the fields treated with water injection the produced water is a mixture of formation water and sea water, and that such water contains residual ions of barium and strontium, the discarding of produced water into the sea presents a potential environmental hazard, since those ions may cause a chemical imbalance in the marine environment.
The mechanism of chemical deposition is to be found in the compounds entrained by water as still soluble substances such as calcium carbonate, etc. which may form over saturated solutions from which salts may precipitate so as to form scales. The precipitation is a function of the initial nucleation followed by the growing of the crystal, and may occur in the producing formation, in the well, equipment, cooling towers, heat exchangers, evaporators as well as any site of water circulation.
In the oil industry, scales are mainly found in the production string and discarding tube, which causes nearly complete plugging of the oil flow by reducing the effective internal diameter of the tube. Scales are also found in separators, where they form a powdery deposit, and in the reservoir bottom. In the marine environment, shellfish take barium for forming shells of barium sulfate instead of calcium carbonate.
Khalil C. N. et al in "Incrusta.cedilla.oes e Antiincrustantes usados na Industria de Petroleo", ("Scales and Anti-Scaling Agents used in the Oil Industry"), 17th Brazilian Congress of Corrosion sponsored by the Brazilian Association of Corrosion, 1993, Rio de Janeiro, R J, Brazil, discuss the kinds of scales occurring in the oil production as well as the working mechanisms of the anti-scaling agents or inhibitors, besides the criteria used in the choice of the inhibitors, among which are the trend to emulsify, compatibility with other products, thermal stability and toxicity.
Preventing the deposition of scales is normally effected with the aid of products the properties of which inhibit the precipitation and/or incrustation of certain compounds which are knowingly insoluble under the conditions of activity, pressure and temperature of the reservoir. These products work in sub-stoichiometric amounts through adsorption into the growing sites of the crystals, as anti-flocculating agents, as well as delayers of the nucleation as well as of the growing and modification of the crystals habit.
U.S. Pat. No. 5,840,658 of the Applicant teaches the use of a scale inhibitor based on anionic polymers, mainly from the class of polyphosphonates and polycarboxylates, of low molecular weight, which acts in a preventive way relative to the cations which are the precipitation precursors. In the condition of production of the well, the potentially scaling water will gradually dissolve the inhibitor which is present in very low concentration, and preferably in a concentration which is close to the minimum effective concentration of the scale inhibitor under the conditions designed for the well.
In spite of the fact that the performance of the inhibitors indicates that prevention of scales may be reached in a safe, efficient, long-lasting and monitorable way, the prevention of scales as effected according to the state-of-the-art technique implies permanent costs in chemical reagents and operation activities.
U.S. Pat. No. 4,723,603 teaches a process for the prevention of plugging caused by the precipitation of insoluble salts which employs a filtration process on a reverse osmosis membrane. The membrane acts in the sense of impeding the passage of precipitate precursor ions present in an injection water to the desired injection water while at the same time allowing the passage of pure water and harmless ions through the membrane. Actually the membrane effects a de-sulfating process since the retained ions are mainly sulfates, which may precipitate when they meet barium and strontium present in the formation water. The membrane is chosen according to the size and pore as well as to the electric charge. Common commercial membranes are those polyamide membranes designed for removing sulfate ions from injection water. In order to meet the requirements of productivity in terms of amounts of injection water, commercial membranes should filter between 8.49 and 84.9 I/m.sup.2 -hr. Normally each unit of injection water filtered by the membrane yields one unit of brine.
Thus, it can be seen that the current methods for the scale prevention/correction in oil production equipment are usually costly either as a result from the permanent use of chemical reagents and operation activities as in the case of the polymeric inhibitors or their cost derives from the equipment itself which is to be used, as in the case of the reverse osmosis membranes, which normally comprise a module of 11 meters high, weighing more than 100 tons and which needs nearly 15 months to be built and mounted.
Therefore, the oil industry needs a secondary oil recovery process which would use the previous and controlled precipitation of the chemical species which act as precipitation precursors, mainly sulfate and barium, followed by the removal of the precipitated ions by filtration, the resulting water being compatible with the producing formation when it is employed as injection water in the enhanced oil recovery. Such high efficiency and low cost process of enhanced oil recovery with injection of treated water, which dispenses with high cost equipment and/or reagents, is described and claimed in the present application.